The chemical conversion coating of aluminum and aluminum alloys is known in the art. Typically, the process is used as a means to protect the metal from harsh service conditions.
The conversion coating is, generally, an intermediate film that exists between the metal surface and the overlying coating, such as paint. When applied uniformly and tightly bonded to an aluminum or aluminum alloy surface, the coating provides a protective layer that resists corrosion and abrasion as well as a substratum to which a final coating, if any, can firmly adhere.
Chemically established oxide films are superior to those that result from the natural oxidation of a surface. Natural oxide films form gradually as a result of the exposure of a metal surface to atmospheric heat and moisture. These films stratify and orient themselves in regular patterns and layers, and they are flaky, lack surface adhesion and thus are unsuitable as either a protective coating or a paint substratum.
Chemical conversion coating processes, by contrast, chemically etch the surface of a metal under controlled conditions to form an oxide film on the surface of the metal, which can then be further processed. A widely accepted chemical conversion coating process for aluminum and aluminum alloys is to treat the surface of the metal with a chromic acid or chromicphosphate solution to chemically form a chromate film. Generally, these chromate films can be efficiently formed and produce superior performance ratings compared to alternative compositions. The films are aligned in random patterns and layers, cohesive in nature and, where sealed, act as an ideal corrosion and abrasion barrier and coating substratum.
Notwithstanding its success in the industry, chromate processes are not without serious disadvantages. First, chromium chromate is a carcinogen, a fact that renders workplace safety difficult to maintain. Second, its high toxicity creates a waste disposal problem, which has resulted in ecological harm in some cases. These problems, indeed, have led to greater governmental control of the use of chromate coatings material, effective in 1995.
Conversion coating processes other than those that use chromium chromate-based solutions to pretreat aluminum and aluminum alloys have been disclosed. For example, formation of protective layers by use of boiling aleionized water and an aqueous alkali metal permanganate composition and a proprietary lithium nitrate bath is known in the art. However, no conversion coating process has achieved the high degree of industry acceptance and superior performance ratings of chromate-oxide fills.
The merits of steam as a post-treatment sealing agent for anodized aluminum and aluminum alloys have long been recognized. In this stage of treating the metal, steam is used not to create a conversion coating, but rather to increase the resistance of an anodic coating to staining and corrosion. The steam, moreover, serves to improve the durability of the color produced in the anodic coating and may impart other desirable properties.
It has been suggested that low density steam at high surface temperatures (around 300.degree. F.) and high pressure could be used to form a high quality aluminum-oxide conversion coating without the negative effects associated with chromate films. Comprehensive study of the process under these narrowly prescribed conditions, however, failed to validate the theory. Indeed, not a trace of the aluminum-oxide film was detected in this or a similar environment. Moreover, any oxide coating that does form on an aluminum workpiece surface will take a considerable amount of time to develop.
Several prior art processes have incorporated additives into the steam, such as accelerating agents or chemical reagents. Examples of these are HNO.sub.3, H.sub.2 O.sub.2, and carboxylic acid. These additives are designed to assist in the development of the oxide layer on the aluminum by speeding up the oxide layer growth. However, as with chromate coatings, the additives may be potentially harmful to the environment. Furthermore, the additives increase the overall cost of the manufacturing process since they must be added to the steam or coated onto the aluminum before subjecting the workpiece to the steam.
In these prior art processes, the purity of steam was not considered to be important. On the contrary, the incorporation of additives, such as accelerating agents, destroys any steam purity that may have originally existed. The inventors of the present invention have determined that the incorporation of these accelerating agents, while increasing the speed of oxide layer development, have the detrimental effect of depositing a residue on the surface of the workpiece. The residue can, in certain instances, degrade the adhesive capability of the oxide layer. For example, when steam is created from standard tap water and applied to an aluminum workpiece, calcium carbonate and/or sodium chloride deposits can result. These deposits severely reduce the adhesive capabilities of the oxide coating. It has also been determined that acceleration agents tend to draw corrosive agents through the coating to the base metal.
Thus, a need for a viable substitute process that eliminates toxic metal solutions still exists. There is also a need for a steam conversion coating process that rapidly produces an acceptable primary oxide film within a low surface temperature range. There is a further need to achieve these goals through a process design that is both economical and without the negative side effects associated with chromate films.
The present invention is a conversion coating process that satisfies these needs.